1. Field of the Invention
The present invention relates to a bushing for the production of round glass fibers.
2a. Technical Considerations
In the forming of glass fibers utilizing modern technology, electrically-heated containers known as bushings, typically constructed of precious metals such as platinum or palladium and alloys thereof, are used. Molten glass is fed into the bushing and flows out through a multiplicity of nozzles or projecting orifices (hereinafter referred to as "tips") carried on what is commonly referred to as a tip plate which typically forms the bottom of the bushing. The flow of glass through the tips is usually driven by the hydrostatic pressure exerted by the molten glass above the tip plate. In some cases, it may be desirable to prepressurize this hydrostatic head by applying a pressurized gas such as air above the glass. As glass exits the tips, natural convection and enhanced radiative heat transfer due to the presence of fin coolers, removes heat from the glass.
A difficulty encountered in the use of platinum alloys for the construction of tip plates is that the wetting angle of the glass on the particular alloy used may be such that molten glass tends to adhere or wet the exterior surface of the tip. Wetting may take place to the extent that the glass from one stream spreads and merges with an adjacent one. This renders further attenuation impossible and the glass must be cleaned from the tip plate so that the process can be restarted. In commercial production, these interruptions are referred to as "breakouts" and their frequency of occurrence must be kept to a minimum in order to maintain a high job efficiency. In recent years, the size of production bushings has increased to the point where tip plates carrying as many as 1,200, 1,800 and even 4,000 or more tips are commonplace, and wetting and subsequent breakout has become more acute.
Since a considerble investment in costly precious metals is required to construct a bushing, it would be advantageous to fabricate as many tips per square inch that can be feasibly accommodated on the tip plate to reduce the quantity of precious metals used. The number of tips or orifices per square inch will hereinafter be referred to as the "packing density" of the tip plate.
The prior art teaches one method for lowering the quantity of precious metals used in bushing construction by eliminating the tips entirely and replacing them with a flat perforated plate having a large number of holes or orifices to accommodate the flow of glass. Unfortunately, as the packing density of the orifices increases, there is a greater tendency to wet the bottom of the plate, as discussed earlier. In an effort to reduce wetting, intersecting grooves may be cut between the orifices at right angles to one another. This restricts the movement of glass across the undersurface of the plate in an attempt to prevent the merger of glass issuing from one orifice with that of another.
Flat perforated plates have also been used to form noncircular fibers. Molten glass is discharged from flat plates having noncircular orifices, and in some instances under high pressure, and the issued fibers are immediately cooled to "freeze" the noncircular shape of the fiber.
In bushings that employ tips as orifices, one is physically limited by the wall thickness of the tips to a theoretical maximum packing density which is often greater than that which can be realized in actual glass fiber production. This is because a limiting threshold is reached as the pitch or spacing between the tips is made smaller and smaller at which point a breakout at one tip may allow molten glass to spread or flood across the surface of the tip plate and interrupt the fiber forming process. Conically shaped or tapered tips have been developed to overcome this difficulty since the area wetted by the glass flowing from each tip is theoretically limited to the bottom surface thereof. But, even when these tips are spaced too closely to one another, glass may flow into the recesses between them by capillary action and again cause a disruption of the forming process.
Nevertheless, it has always been the usual practice of the industry to fabricate individual tips in the form of round conical cylinders having circular cross-sections. Examples are disclosed in the book entitled The Manufacturing Technology of Continuous Glass Fibers by K. L. Lowenstein, published by the Elsevier Scientific Publishing Company, New York, 1973 at pages 94-95. It has also been observed in both the patent and scientific literature that the production of round fibers is usually illustrated as originating from tips having a round cross-section.
It would be advantageous to be able to produce round fibers with bushings having a higher packing density while at the same time be able to maintain the processing stability required to prevent flooding of the bushing and subsequent breakout.
2b. Patents of Interest
U.S. Pat. Nos. 2,489,508 to Stalego and 3,607,185 to Andrysiak disclose forming glass fibers using a bushing having tips that share a common sidewalls. The outlet end of the tips formed by their common walls may be noncircular and is much larger than the inlet end of the tip.
U.S. Pat. No. 3,475,147 to Stalego discloses a method and apparatus for dividing a source of heat softened mineral material into a plurality of smaller streams. The tips generally have a circular inlet end and a noncircular outlet end.
U.S. Pat. No. 4,343,635 to Kim et al. discloses a glass fiber forming apparatus wherein the orifice plate of the bushing has a plurality of orifices separated by grooves in lower surface of the plate to reduce wetting of the plate by the molten glass.
U.S. Pat. Nos. 3,979,195 to Strickland; 4,622,054 and 4,636,234 to Huey et al.; 4,666,485 to Huey and 4,759,784 to Shono et al. all disclose the use of flat forming plates with noncircular openings to form noncircular glass fibers. In each, molten glass exits through the openings in the plate, and in some instances under high pressure, and is immediately cooled to maintain its noncircular configuration. Although these references recognize that the surface tensions in molten glass may tend to change the configuration of the glass stream upon exiting the forming plate, they do not recognize or appreciate the problem associated with increasing the temperature of the glass in order to lower the glass viscosity, namely flooding of the forming plate and subsequent breakout. Increasing the glass temperature as inferred in the references would result in the glass flooding the plate, thus disrupting the entire glass fiber forming operation. Furthermore, while the effects of surface tension forces are known in the art, these patents disclose methods to avoid these effects. Surface tension forces have never been used to produce round fibers from a noncircular tip. As it will be shown, several advantages can be gained by doing so.